The present invention relates to a semiconductor device; and, more particularly, to a ferroelectric capacitor of semiconductor device including ferroelectric thin film as a capacitor dielectric film. The present invention also relates to a method for fabricating the same.
Ferroelectric Random Access Memory (FeRAM) is a nonvolatile memory device using ferroelectric material such as (Sr,Bi)Ta2O9 (SBT) and PbZrxTixxe2x88x921O3 (PZT), as capacitor dielectric material. It is promising because it memorizes its stored information even at the time of cutting off power supply and equals to the conventional Dynamic Random Access Memory (DRAM) in view of operating speed.
FIG. 1 is a sectional view showing a ferroelectric capacitor fabricated in accordance with the conventional method. A platinum film 11 for bottom electrode is formed with a thickness of 1500xcx9c3000 xc3x85 on a lower layer 10 formed by predetermined processes. 1500xcx9c3000 xc3x85 of a SBT thin film 12 and 1500xcx9c3000 xc3x85 of a platinum film 13 for upper electrode are then formed on the bottom electrode, in turn. The films are selectively etched to form a capacitor.
In the case of forming the SBT thin film 12 by sol-gel method or metallorganic material chemical vapor deposition, conventionally, spin coating or deposition, baking and rapid thermal process (RTP) of SBT precursor are performed, in turn, two or three times to form the SBT film with a desired thickness (1500xcx9c3000 xc3x85 ). The treated SBT film is furnace-annealed under oxygen atmosphere at 800xc2x0 C. for 1 hour. Here, the RTP is to thermal-treat the SBT film under oxygen atmosphere at a temperature of 700xcx9c750xc2x0 C. during short time (for example, 30 seconds to 2 minutes) in order to form fine crystalline nuclei of SBT from the amorphous SBT film 12 after the spin coating and baking. The furnace annealing process is to form the SBT film 12 with sufficient ferroelectric characteristics by growing the SBT crystalline grains to a size of 1000xcx9c2000 xc3x85.
The crystalline structure of the SBT film 12 is a sandwich structure in which a (Sr,Bi)Ta2O9 layer of perovskite structure is inserted between two Bi2O3 layers. In the case of forming the SBT film 12 using the precursor such as metal 2-ethylhexanoate by sol-gel method or metallorganic chemical vapor deposition, the crystalline grain of the formed SBT film by a series of processes including spin coating or deposition, baking, RTP and furnace-annealing is rod-like crystal with 1000xcx9c2000 xc3x85 size. Therefore, there are some drawbacks that the surface of the SBT film 12 is very rough and the SBT film is not dense. Also, the roughness of the surface allows the SBT film to become thin locally, thereby, increasing leakage current and lowering breakdown voltage.
It is, therefore, an object of the present invention to provide a ferroelectric capacitor of semiconductor device in which the surface roughness of the SBT thin film is lower and the film is denser (namely, the film quality is better) in comparison with the conventional SBT film.
It is another object of the present invention to provide a method for fabricating the same ferroelectric capacitor.
In accordance with an aspect of the present invention, there is provided a ferroelectric capacitor of semiconductor device, which comprises a conductive film for bottom electrode on a predetermined lower layer; a first (Sr,Bi)Ta2O9 thin film made of crystalline grains of (Sr,Bi)Ta2O9, on the bottom electrode; a second (Sr,Bi)Ta2O9 thin film made of crystalline nuclei of (Sr,Bi)Ta2O9, on the first (Sr,Bi)Ta2O9 film; and a conductive film for upper electrode on the second (Sr,Bi)Ta2O9 film.
In accordance with another aspect of the present invention, there is provided a method for fabricating a ferroelectric capacitor of semiconductor device, which comprises the steps of forming a conductive film for bottom electrode on a predetermined lower layer; forming a first (Sr,Bi)Ta2O9 thin film made of crystalline grains on the bottom electrode; a second (Sr,Bi)Ta2O9 thin film made of crystalline nuclei on the first (Sr,Bi)Ta2O9 film; and forming a conductive film for upper electrode on the second (Sr,Bi)Ta2O9 film.
The SBT thin film of the present invention has double layer structure, in which each layer is different in detail crystalline structure of the thin film from each other fabricated by the following method under different condition of thermal treatment. SBT precursor is spin-coated, baked, thermal-treated with RTP and furnace-annealed, in turn, to form a first SBT thin film made of conventional crystalline grain structure. On the first SBT film, SBT precursor with lower viscosity is then spin-coated, baked and thermal-treated with RTP, in turn, to form a second SBT thin film made of only crystalline nuclei. Therefore, the present invention can improve the surface roughness and quality of the SBT thin film.